It is well known to use phosphors to convert blue light emitted from LEDs into longer wavelengths in order to affect the overall color of the light emitted from the LED package. See, e.g., U.S. Pat. Nos. 6,613,247 and 6,653,765. Such LEDs are generally referred to as phosphor-conversion LEDs (pc-LEDs). One very important application for pc-LEDs is the generation of white light in which a portion of the blue light emitted by the LED is converted into a yellow light by a phosphor embedded in the epoxy resin that encapsulates the LED. The combined effect of the remaining unconverted blue light and the converted yellow light is to produce an overall white light output from the pc-LED. Additional phosphors can be added to produce a warmer white color with a higher color rendering index (CRI). The preferred phosphor for white-emitting pc-LEDs is a yellow-emitting, cerium-activated yttrium aluminum garnet phosphor which can be represented by the formula Y3Al5O12:Ce3+, (YAG:Ce). The Ce3+ luminescent species in the yttrium aluminum garnet (YAG) matrix absorbs in the blue region of the visible spectrum (420-490 nm) and re-emits the absorbed energy in the yellow at about 570 nm.
However, the use of phosphor powders in pc-LEDs is not without problems. In particular, it can be difficult to obtain uniform phosphor layers over the LED source leading to unacceptable color variations between LEDs. In addition, the phosphor particles embedded in an epoxy resin scatter the blue light emitted from the LED, which can reduce efficiency.
Another means of achieving a similar result in pc-LEDs is to use a solid, sintered ceramic converter instead of phosphor powders. Such solid, monolithic converters have at least two distinct advantages over phosphor powders. First, luminescent ceramic converters can be made in defined shapes and uniform thicknesses to provide better consistency and color control in manufacturing. Second, they can be made translucent which can reduce scattering losses and improve extraction efficiency. Examples of such luminescent ceramic converters are described in U.S. Pat. No. 7,554,258, U.S. Patent Application Publication 2007/0126017 and International Patent Application Publication No. WO 2006/087660.
Similar to the phosphor powders used in the pc-LEDs described above, the ceramic converters in white pc-LEDs are used to covert the light from the blue LED into a yellow light in order to produce an overall white light. Typically, the luminescent ceramic converter is a thin, flat plate approximately 1 mm square and less than 200 micrometers in thickness. The converter is fixed to the surface of the LED die so that the converter is in close proximity to the light-emitting surface. Like the phosphor-converted LEDs described above, the converter is preferably comprised of cerium-activated yttrium aluminum garnet, Y3Al5O12:Ce3+. Some gadolinium may also be incorporated into the YAG structure to slightly shift the emitted color more toward the red.
One targeted application for pc-LEDs using luminescent ceramic converters is automotive headlamps where the color uniformity of the beam projected on the road is an important consideration. In order to achieve this, the color of the light output by the pc-LED package must remain relatively constant as the viewing angle is changed. This is problematic for pc-LEDs that use highly translucent or transparent ceramic converters as the distances through which the blue light rays must travel within the converter become greater the more the viewing angle deviates from the surface normal. As a result, light emitted at an angle of 0° with respect to the surface normal will be bluer than light emitted at angles greater than 0° with respect to the surface normal.
One solution to reduce the difference in the angular color shift is to create a longer path length for all light rays by introducing scattering sites inside the ceramic converter. This can be achieved by leaving pores in the ceramic material as described in International Patent Application Publication No. WO 2007/107917 A2. However, as with phosphor powders, excessive scattering can result in an unacceptable loss of extraction efficiency. Moreover, the effectiveness of the scattering will be determined by both the number and size of the pores in the ceramic. If the number is too large, the light will be largely absorbed by internal scattering and the overall LED light output reduced. Light output is also reduced if the pore size distribution is not within a narrow optimal zone. In particular, it has been reported that the efficacy is optimal with 800 nm sized pores and that the efficacy drops off rapidly below 500 nm and steadily above 1000 nm. Achieving the optimal pore size distribution in a luminescent ceramic converter is difficult due to the thermodynamic and kinetic aspects of ceramic processing.